1. Field of the Invention
The present invention relates to the field of dental prostheses manufacturing and in particular to an improved method and firing tray facilitating purging of the furnace during the firing to minimize discoloration of the dental prostheses by contamination in the furnace.
2. Description of the Prior Art
In the manufacture of dental prostheses, a layer of ceramic material is generally placed over a base of precious metal and the composite is baked in a furnace. The teeth, which are generally hollow, are placed on pegs, which resemble inverted cones, and which in turn sit on trays of ceramic materials. Several teeth, either individually or as bridges, may be fired simultaneously as a single tray load.
As the price of precious metals has risen, dental labs have sought to use semi-precious metals, e.g, palladium-silver, as substitutes for gold. Unfortunately, the use of such metals results in discoloration of the teeth. This discoloration has been termed "greening" by those skilled in the art. The greening process is not at all well understood, but it is generally believed, by the routineers in the field, that semi-precious metal alloys emit a vapor, such as silver vapor, which contaminates the tooth directly and/or contaminates the interior of the furnace. This contamination produces aesthetically undesirable results by randomly staining the surface of the porcelain during the firing cycle.
There are currently in use, with varying degrees of success, a number of technical procedures for reducing the incidence of greening, including scrubbing, purging the furnace, etc. None of these known procedures, however, has been totally effective.
The most important of the prior procedures involves the use of carbon as an absorbant. After a number of firing cycles involving dental prostheses which comprise semi-precious metals, the furnace becomes contaminated. The carbon is placed in such an empty, contaminated furnace and the furnace is operated through a complete firing cycle. The carbon acts to absorb contamination from the furnace and the furnace is thus "cleansed" or "purged" so that it becomes suitable for subsequent firing of teeth. Such known method is generally effective; however, it is inconvenient and inefficient since the furnace and the technician periodically must be occupied with the non-productive firing cycles. Also, these previous procedures create a cyclic nature to the greening problem and presumably the greening of the procelain of the dental prostheses produced in the furnace just prior to purging of the furnace is more severe than it is just subsequent to the purging operation. Thus, the quality of the teeth produced varies with time. That is to say, between cleaning cycles the furnace gets dirtier and dirtier.